Current detector and current measuring apparatus including such detector with temperature compensation

ABSTRACT

A current sensing device and a residual current detection device are described having a temperature compensation capability so that residual current can be directly measured to a high degree of precision in the background of a high load current. The residual current device comprises a plurality of resistive shunts for connection in respective ones of a plurality of lines through which current can flow to and from a load, and a detector means sensitive to the voltage developed across each of the shunts to detect any imbalance between the currents flowing through the shunts.

FIELD OF THE INVENTION

This invention relates to a current sensor with a temperaturecompensation capability intended for use in an electrical apparatus suchas a residual current correction device, a current meter or a powermeter.

DISCUSSION OF THE BACKGROUND ART

It is an aim of the present invention to provide a current sensor ineconomical form which includes temperature sensing means forfacilitating temperature compensation.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a current sensorcomprising a metallic link member having two end portions of conductivematerial and an intermediate portion interconnecting the end portions,said intermediate portion being formed of a resistive material, and anintegrated circuit analog to digital converter mounted on said linkmember, said converter having analog input terminals connected torespective ones of said two end portions and digital output terminalsfor connection to a processing apparatus, wherein a temperature sensoris provided on or within said intermediate portion.

The temperature sensor is preferably an electronic semiconductortemperature sensor and may be mounted directly onto the intermediateportion with a thermally suitable conducting glue. The temperaturesensor may be built into (i.e. integrated) into the integrated circuitanalog to digital converter in which case it will form part of asemiconductor die mounted directly onto the intermediate portion.

Embodiments of the invention have the advantage that the temperaturesensor will follow the temperature of the intermediate portion veryclosely. It is therefore possible to compensate for changes in theresistance of the intermediate portion resulting from temperaturevariations as the current flowing therethrough changes.

Conveniently, the converter is attached to the intermediate portion by alayer of electrically insulative adhesive material and the analog inputterminals of the converter are connected to the end portions by wirebonds.

The converter preferably includes a delta-sigma modulator which providesa high frequency one-bit digital data. One or more decimation filteringstages may be included in the converter.

The converter may also have a voltage reference terminal for connectionto a reference voltage source, the converter operating to providedigital output signals respectively representing the current flowingthrough said intermediate portion and digital output signalsrepresenting the voltage on one of said end portions.

Embodiments of the invention may be advantageously employed in residualcurrent devices. Conventionally, residual current is detected utilisinga current transformer having primary windings through which, in the caseof a single phase device, load current flows in opposite directions sothat if the return current is different from the outwardly flowingcurrent because of current leakage an output current signal is inducedin a secondary winding of the transformer. In the case of a multi-phasedevice, primary windings of the transformer are connected in all of thephase lines and the neutral line. In normal situations, when there is nocurrent leakage, the net current induced in the secondary winding iszero and therefore no output is detected.

Sophisticated materials have been developed for the core of the currenttransformer, which enable considerable accuracy to be obtained when thecurrents flowing in the primary windings are substantially sinusoidal.However, switch mode power supplies are often used for computers andother equipment and there is an increasing tendency for such equipmentto cause dc offsets in the currents. Such developments have madedetectors utilising current transformers less reliable and prone tofalse tripping or failure to detect a dc current leakage.

This is a particular problem in the case of directly actuatedelectro-mechanical devices, where the current transformer secondarywinding actually drives an actuator. The situation is not much improved,when including an electronic detection and amplification means connectedto the secondary winding, as there are still problems with highfrequency transients and dc offsets. A very small dc current level cancause the core to saturate thereby seriously impairing the ability ofthe detector to detect current leakage.

It is also an aim of the present invention to provide a residual currentdetection device in which the above mentioned problems are substantiallyovercome in a simple and efficacious manner.

In accordance with the invention there is further provided a residualcurrent detection device comprising a plurality of resistive shunts forconnection in respective ones of a plurality of lines though whichcurrent can flow to and from a load, and detector means sensitive tothevoltage developed across each of the shunts to detect any imbalancebetween the currents flowing though the shunts, wherein a temperaturecompensation means is provided for facilitating compensation forfluctuations in shunt resistance with variations in temperature.

In preferred embodiments, the temperature compensation means is atemperature sensor provided on or within each of said plurality ofresistive shunts.

Preferably, the detector means comprises an analog to digital converterfor each shunt and a processor for receiving the digital signals fromthe converters and determining whether a current imbalance exists. Inthis case, the temperature sensor may be built into (i.e. integrated)into the analog to digital converter. The temperature sensor ispreferably an electronic semiconductor temperature sensor mounteddirectly onto the shunt with a thermally conducting glue.

Each shunt preferably takes the form of a composite strip havingconductive portions at its ends and a resistive portion interconnectingthe conductive portions. Such composite strips can be mass producedinexpensively to very high tolerances which makes them extremelysuitable for this purpose.

The analog to digital converter for each shunt may include a delta-sigmamodulator, which generates a high frequency single digital data streamwhich is converted by decimation filtering to a multibit digital datastream at a lower frequency.

The analog to digital converter for each shunt is preferably connectedto the processor through an isolation barrier so that the converter canfloat at the voltage level of the shunt which it serves. The decimationfiltering may be effected entirely in the converter, entirely in theprocessor or split between the converter and the processor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view of an example of the inventionas applied to a single phase device,

FIG. 2 is a block diagram of an another example of the invention asapplied to a three phase device,

FIG. 3 is a perspective view showing one of the current sensing devicesembodying the present invention,

FIG. 4 is a sectional view of the current sensing device of FIG. 3,

FIG. 5 is an elevation of the device of FIG. 3,

FIG. 6 is a block diagram of a simple form of the electronic circuit ofa single current sensor device,

FIG. 7 is a block diagram of an alternative form of the electroniccircuit,

FIG. 8 is a block diagram of yet another form of the electronic circuit,and

FIG. 9 is a block diagram of a form of the electronic circuit whichincorporates a temperature sensor in accordance with and embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the device shown in FIG. 1, a substrate 10 supports two compositeconductor strips 11, 12. Each of these includes end portions 13 ofcopper and an intermediate portion 14 of a resistive material such asmanganin. The strips are formed by slicing up a sandwich formed byelectron beam welding the copper portions to opposite sides of themanganin portion. The shunts formed by the resistive portionsmanufactured by this method can have a nominal resistance of 0.2 mΩ to atolerance of less than 5%. If the two shunts 14 used on one device arepressed from adjacent portions of the sandwich stock, they are matchedto within 2%. Calibration of the shunts built into a unit at twodifferent temperatures can virtually eliminate shunt errors. In thisway, at least two temperature measurements are made. Two temperaturemeasurements are taken because the difference in shunt A from shunt B islinear when the devices are adjacent to one another.

However, it is desirable to provide for direct compensation fortemperature fluctuations arising from current fluctuations especially ina single current detector. The resistivity of precision resistancematerials does not change with temperature. Compared to pure metals suchas copper or aluminium, with Temperature Coefficient of Resistance (TCR)values close to 4000 ppm/DegC., the TCR values of Manganin or Zeraninare more than a factor of 400 better—but still not zero.

In reality, the plot of Resistance Vs Temperature (R(T)-curve) is notstrictly linear and it is common practice to describe the curves by athird order polynomial. In general this is:

R(T)=R_(o)*(1+a_(o)*T+b_(o)*T²+c_(o)*T³) where T=Temperature in DegC. &R_(o)=Resistance at 0 DegC..

At a more practical Reference Temperature of 20 Deg C.. we can rewritethis as R(T)=R20*[1+a_(o)*(T−20)+b_(o)*(T−20)²+c_(o)*T(−20)³]

The typical curves for the resistance materials Manganin and Zeranincurves are determined by the main composition of the alloys and varyvery little from batch to batch. The production spread is less than 5 to10 ppm/DegC.. These slight differences in the TCR value can be expressedin a tiny change of the first order coefficient “a” in the aboveequation and the second and third coefficients are basically notchanged. For example a dR(T)/R20-curve for different batches is justrotated around the 20 DegC. point and the curve itself is unchanged.This explains the calibration of the shunts at two differenttemperatures mentioned above.

However in accordance with the present invention, it may be desired tocalibrate each individual shunt in an RCD OR in the case of the CurrentSensor (Single Shunt) we could calibrate it separately. Varying as athird order polynomial at least 4 if not more points for a goodcalibration would be needed.

As described by the above equations if we know the temperature and ourRef resistance R20 for example we can with a suitable number of pointsfind the coefficients and calibrate the shunt.

In the preferred embodiment, the temperature sensor itself is built into(integrated) and is a part of the ASIC which includes the analogue todigital converter ADC. In other words the temperature sensor will be anelectronic semiconductor temperature sensor in the ADC. The ADC ismounted as a semiconductor die directly onto the shunts with a suitablethermally conducting glue and will therefore track the temperature ofthe Manganin (shunt) very accurately.

Preferably, the temperature sensor is input and sampled via the voltagemodulator. It could have its own modulator (see RCD FIG. 9 showing theadded temperature channel) or be multiplexed into the voltage channel(see FIG. 2).

Is possible to avoid having to make several stable temperatures to makethe measurements. Instead we propose to measure at 20 Deg C. for exampleand then apply a known current which heats up the shunts and makeseveral measurements during this process until the shunt arrives at itsnew steady state temperature as a result of the applied current.

In the example shown in FIG. 1, there is a separate signal preprocessingASIC 15 mounted on each of the shunts 14 and connected to the copper endportions 13 of the associated conductor strips. The two ASICs 15 areconnected to via an isolation transformer array 16 to a main processor17. The ASICs 15 operate to convert the two voltages across the shuntsinto a digital signal stream which is communicated to the processor 17via the isolation transformer array. The main processor is programmed toprovide a drive signal to a trip actuator 18.

The actual preferred structural configuration of the current sensors isshown in FIGS. 3 to 5. These show leads 40 connecting two analog inputterminals of the ASIC to the two copper end portions 13. Other leadsconnect other terminals of the ASIC 15 to a lead frame 40 a by means ofwhich all other external connections are made. FIG. 5 shows in dottedlines a block 42 of encapsulation material and FIG. 4 shows anelectrically insulative adhesive layer 41 by means of which the ASIC isattached to the intermediate portion 14, which may be of manganin orzeranin of the composite strip 14, 15. The strips are formed by slicingup a sandwich formed by electron beam welding of the copper bars toopposite sides of a manganin bar. The temperature sensor is preferablyintegrated within the ADC of the ASIC 15.

FIG. 6 shows that within the ASIC 15 there is provided a singledelta-sigma modulator 15 a. There is also an analog input circuit whichhas its input terminals connected to the copper end portions 13. Theoutput of the ASIC 15 in this case consists of a high frequency one-bitdata signal train. In use, the ASIC output is connected via atransformer or other isolation barrier 16 to a processor 17. Theprocessor in this arrangement is configured to carry out one or moredecimation filtering operations to convert the one-bit signal streaminto a multi-bit value at a lower frequency.

The processor 17 may typically be configured to receive signals from aplurality of the detectors and to sum these signals to ascertain whetherthe current flows through the detectors are balanced. Such anarrangement can be used for residual current correction allowing anactuator to trip a switch if an unbalanced condition is found to exist.The processor 17 may alternatively or additionally compare theinstantaneous current level with a trip level so that overcurrenttripping can be controlled.

FIG. 2 shows in rather more electrical detail a three phase device. Inthis case there are four shunts 14, one in each phase line and a fourthin the neutral line. The ASICs 15 of FIG. 1 are shown as four separateblocks 20, 21, 22, and 23, and there is a power supply unit 24 whichdraws power from the phase lines on the mains side of the shunts 14 andprovides controlled voltages to the processor 17. Power is supplied tothe four blocks 20 to 23 via isolation barriers 25 which make up thearray 16. Each block of the ASIC includes an analog to digital converterin the form of a delta-sigma modulator which provides a high frequencyone bit digital data stream. A multiplexer may be included in eachconverter so that the converter can provide to the processor, throughthe respective isolation barrier, signals representing both current inthe associated shunt and the voltage at one end of it. The processoruses these signals to monitor the current in each shunt and to operatethe actuator 18 if an imbalance occurs.

It will be noted that the voltage sensing connections to the ASICs aremade via resistor chains connected between each phase line and theneutral. Each such resistor chain comprises an outer pair of precisionresistors of relatively low ohmic value and an intermediate resistor ofrelatively high ohmic value. These resistor chains allow the RCD to beprovided with an independent reference. If the neutral ADC is taken asthe selected system reference, then the operating software of the mainprocessor can use the multiple signals derived from the several resistorchains to calibrate each phase against the neutral reference.

The CPU is programmed to carry out the necessary calculations todetermine the existence of an imbalance and can determine the true RMSvalue of the residual current, which conventional devices fail to docorrectly particularly in the case of non-sinusoidal current waveforms.The CPU may be programmed to enable it to determine from the data itreceives whether a particular event is, in fact, an unacceptable leakagemore reliably than conventional devices. For example, the CPU can takeinto account the historic performance of the unit when setting theleakage current threshold and may ignore events which have arecognisable “signature”. In this way improved tolerance to nuisancetripping can be obtained

Decimation filtering of the high frequency one bit data stream isrequired to reduce each data stream to a multi-bit digital signal at apredetermined sample frequency. By way of example, each current signalmay be a 23-bit signal at a sample rate of 64 times the mains frequency,but lower resolution at lower sample rates can be employed whennon-linear, rather than linear conversion is acceptable. The decimationfiltering is typically a function of the processor, filtering of thefour data streams being executed simultaneously so that sample valuesare derived for all four shunts simultaneously. A circuit employing suchan arrangement is shown in FIG. 6 as described above.

In an alternative embodiment as shown in FIG. 7, one or more stages ofthe decimation filtration may be executed by hardware included withinthe ASIC. This includes a serial output driver 15 b to transmit the bitsof the multi-bit digital signal produced by the filtration stage 15 cserially to the processor. Multi-bit digital words are transmittedserially across the isolation barriers instead of a one-bit signalstream. The filtration stages may be split between the ASIC and theprocessor. With this arrangement, the configuration of the processor canbe simplified as part or all of the decimation filtration operation iscarried out in the ASIC.

Where current and voltage are both to be monitored as in the systemshown in FIG. 2, the circuit 15 may be as shown in FIG. 8 with separatemodulations and filtering components for the two signal streams and acommon serial interface. Alternatively separate serial interfaces may beemployed. The ASIC of FIG. 8 has a further analog input which can beconnected to a reference voltage source. Two analog input stages 21 aand 21 b are present and these feed signals to two independentdelta-sigma modulators 15 d, 15 e. As shown, there are two independentdecimation filtration stages 15 f, 15 g for the two one-bit digitalsignal streams. The outputs of the stages 15 f, 15 g may, as shown, beconnected to a common serial output stage or (not shown) separate serialoutput stages may be provided.

It will be appreciated that the arrangement of FIG. 8 may be modified bythe omission of the two filtration stages 15 f, 15 g where allfiltration is to be carried out by the processor.

Where voltage as well as current is monitored by the processor, precisecalibration of the shunts can be achieved. This allows more accuratedetermination of the current balance in RCD applications. Moreover, asvoltage and current are both being monitored to a high level ofprecision, accurate power consumption metering can be obtained.

Where the devices of the invention are used in RCD and overcurrent tripsystems, the processor can be programmed to recognise the transientswhich may occur when loads are switched in and out of circuit to avoidfalse tripping. Many other convenient functions can be programmed intothe processor, made possible by the high precision of the currentmeasurements capable of being carried out.

FIG. 9 shows an arrangement similar to the one of FIG. 8 except for theaddition of a temperature sensor in accordance with embodiments of thepresent invention. The temperature sensor is input and sampled via thevoltage modulator. The sensor could have its own modulator (FIG. 9) orbe multiplexed into the voltage channel as mentioned above.

The arrangements described enable very accurate detection of currentimbalance to be effected even in the presence of switching transientsand DC offsets. The problems which arise from potential saturation ofthe current transformer core are avoided completely.

Since the CPU receives actual line current and voltage data from each ofthe blocks 20 to 23, it can be programmed to perform other calculations,such as current limit and power consumption. Thus an RCD deviceconstructed as described above can also provide the functions of aconventional circuit breaker and/or those of a power consumption meterwithout any additional sensing or analog-to-digital components beingrequired.

What is claimed is:
 1. A residual current detection device comprising aplurality of resistive shunts for connection in respective ones of aplurality of lines through which current can flow to and from a load,and detector means sensitive to the voltage developed across each of theshunts to detect any imbalance between the currents flowing through theshunts, the detector means comprising a converter in the form of anintegrated circuit mounted on and electrically connected to each of theresistive shunts and temperature compensation means, including atemperature sensor provided on or within a corresponding one of saidresistive shunts or in said integrated circuit, for facilitatingcompensation for fluctuations in shunt resistance with variations intemperature.
 2. A device as claimed in claim 1, in which the convertercomprises an analog to digital converter for the corresponding one ofsaid resistive shunts and the detector means includes a processor forreceiving digital signals from the converter for each of said resistiveshunts and determining whether a current imbalance exists.
 3. A deviceas claimed in claim 2, in which the analog to digital converter for eachof said resistive shunts includes a delta-sigma modulator which producesa high frequency single bit digital stream which is converted bydecimation filtering into a multi-bit digital data stream at a lowerfrequency.
 4. A residual current detection device according to claim 2,said processor is adapted to provide at least one selected from thegroup consisting of: power metering, circuit breaking and arc faultprotection.
 5. A device as claimed in claim 1, in which each of saidresistive shunts takes the form of a composite strip having conductiveportions at its ends and a resist portion interconnecting the conductiveportions.
 6. A device as claimed in claim 1, in which the correspondingone of said resistive shunts includes two copper end portions and saidintegrated circuit has analog input terminals connected by lead wires tothe two copper end portions of the corresponding one of the resistiveshunts.
 7. A device as claimed in claim 6, in which the integratedcircuit also has a terminal connected to a voltage reference source andincludes a second converter for providing a digital stream dependent onthe voltage on one of the copper end portions of the corresponding oneof the resistive shunts.
 8. A device as claimed in claim 1, wherein thetemperature compensation means comprises said temperature sensor in saidintegrated circuit.
 9. A residual current detection device according toclaim 8, wherein the temperature sensor is an electronic semiconductortemperature sensor mounted directly onto said intermediate portion witha thermally suitable conducting glue.
 10. A device as claimed in claim1, in which the converter comprises an analog to digital converter foreach of the resistive shunts.
 11. A current sensor comprising a rigidmetallic link member having two end portions of conductive material andan intermediate portion interconnecting the end portions, saidintermediate portion being formed of a resistive material, and anintegrated circuit analog to digital converter mounted on saidintermediate portion, said converter having analog input terminalselectrically connected to respective ones of said two end portions anddigital output terminals for connection to a processing apparatus,wherein a temperature sensor is provided on or within said intermediateportion or in the integrated circuit analog to digital converter mountedonto said intermediate portion.
 12. A current sensor as claimed in claim11 in which the converter is attached to the intermediate portion bymeans of a layer of electrically insulating adhesive.
 13. A currentsensor as claimed in claim 12 in which the analog input terminals of theconverter are connected to the end portions by means of wire bonds. 14.A current sensor as claimed in claim 11, in which the converter has avoltage reference terminal for connection to a reference voltage sourceand said converter operates to provide digital output signalsrepresenting the current through said intermediate portion and digitaloutput signals representing the voltage on one of the end portions. 15.A current sensor as claimed in claim 11, in which said converterincludes a delta-sigma modulator which provides a high frequency one-bitdigital data steam.
 16. A current sensor as claimed in claim 15 in whichthe converter also includes at least one decimation filter stage.
 17. Acurrent sensor according to claim 11, wherein the temperature sensor isan electronic semiconductor temperature sensor mounted directly ontosaid intermediate portion with a thermally suitable conducting glue. 18.A current sensor according to claim 11, wherein the temperature sensoris integrated into the integrated circuit analog to digital converter.19. A current measurement apparatus comprising: at least one currentsensor comprising: a rigid metallic link member having two end portionsof conductive material and an intermediate portion interconnecting theend portions, said intermediate portion being formed of a resistivematerial, and an integrated circuit analog to digital converter mountedon said intermediate portion, said converter having analog inputterminals electrically connected to respective ones of said two endportions and digital output terminals, wherein a temperature sensor isprovided in the integrated circuit analog to digital converter mountedonto said intermediate portion; and a processor circuit connected to thedigital output terminals of each of said at least one current sensor toreceive and process digital signals received from said at least onecurrent sensor, in order to determine whether a current imbalanceexists.
 20. A current measurement apparatus as claimed in claim 19 inwhich the processor circuit is configured to carry out one or moredecimation filtering operations on the received digital signals.
 21. Acurrent measurement apparatus according to claim 19, said processorcircuit is adapted to provide at least one selected from the groupconsisting of: power metering, circuit breaking and arc faultprotection.